1. Field of the Invention
This invention relates to a sheet thickness measuring apparatus for measuring the thickness of a sheet, such as a paper sheet, an opaque plastic film, a metal foil, or the like with a magnetic field utilization sensor and an optical sensor.
2. Description of the Prior Art
A conventional sheet thickness measuring apparatus of this type measures, as schematically shown in FIG. 5 of the accompanying drawings, the thickness of a sheet 2 by scanning a sensor assembly 5 including a magnetic field utilization sensor 3, such as a magnetic sensor or an eddy current sensor or the like and an optical sensor 4 along the sheet 2 placed on a metallic backup plate 1. In principle, the distance l.sub.1 to the surface of the metallic backup plate 1 is measured by the magnetic field utilization sensor 3, the distance l.sub.2 to the upper surface of the sheet 2 is measured by the optical sensor 4, and the thickness t of the sheet 2 is obtained at the point by the following equation. EQU t=l.sub.1 -l.sub.2
However, when the thickness of the sheet is actually measured by such a conventional sheet measuring apparatus, it can be measured in accuracy up to approx. 1/100 mm without problem, but there arises a trouble when it is measured in accuracy up to approx. 1/1000 mm. This trouble remarkably occurs particularly when it is measured while moving the sensor with respect to the metallic backup plate.
For example, when the sensor assembly 5 is scanned on the surface of the metallic backup plate 1 on which the sheet 2 is not placed and the output characteristics of the magnetic sensor 3 and the optical sensor 4 at that time are recorded, the relationships between the magnetic output, the optical output and the scanning positions become as shown in FIG. 6. FIG. 6(A) shows an example of the output characteristic of the magnetic sensor 3, and FIG. 6(B) shows an example of the output characteristic of the optical sensor 4. Since the magnetic sensor 3 and the optical sensor 4 measure the same surface, the output characteristics of the magnetic sensor 3 and the optical sensor 4 must become the same configuration in this case, but, as apparent when FIG. 6(A) is compared with FIG. 6(B), the output characteristic of the magnetic sensor 3 is varied largely and complicately as compared with that of the optical sensor 4. The causes can be presumed from various experiments, references and studies, and it is considered that the local magnetization and irregular quality of the materials of the metallic backup plate seem to be causes of an error of the magnetic sensor. If a magnetic material is employed as the metallic backup plate, when a magnet is attached partly and locally magnetized (several gausses) and scanned, the output of the magnetism utilization sensor exhibits a variation of several tens microns. When the surface of the backup plate is examined by a gaussmeter, there is a position exhibiting a large variation even if it exhibits no variation in the output of the sensor (where the resolution of the gaussmeter is 0.1 gauss). Similar problem occurs even if a non-magnetic material is employed as the metallic backup plate and an eddy current type magnetic sensor is used. This is because the error of the magnetic field utilization sensor is considered to disturb to raise the measuring accuracy of the sheet thickness measuring apparatus of this type.
An object of this invention is to provide a sheet thickness measuring apparatus which can eliminate the above described conventional problems.